1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing it, more particularly to a high electron mobility transistor having a metal-insulator-semiconductor structure and a field plate.
2. Description of the Related Art
A high electron mobility transistor (HEMT) is a type of field-effect transistor in which current flows in a two-dimensional electron gas (2DEG). One known HEMT structure has a substrate including a doped gallium nitride (GaN) electron channel layer and an aluminum gallium nitride (AlGaN) electron supply layer. Source, drain, and gate electrodes are formed on the surface of the AlGaN electron supply layer. A 2DEG layer is formed within the electron channel layer by piezo polarization and/or spontaneous polarization of the heterojunction interface between the electron channel layer and the electron supply layer. The electron supply layer has low resistance in its thickness direction and high resistance in the transverse direction, so current flowing between the source and drain electrodes moves in the 2DEG layer. HEMTs of this type combine high switching speeds with high-temperature and high-power operating capabilities, making them promising candidates for high-performance electronic devices.
During alternating-current operation of this type of HEMT, however, a negative charge builds up on the surface of the electron supply layer, causing a so-called current collapse in which the maximum drain current is less than the maximum drain current in direct-current operation.
It is known that the current collapse can be reduced by a silicon nitride (SiN) surface passivation film fifty to one hundred nanometers (50-100 nm) thick, for example, formed on exposed surfaces of the substrate and the source, gate, and drain electrodes. The presence of an SiN passivation film on the upper surface of the electron supply layer of an HEMT, however, has an adverse effect on the source-drain breakdown voltage of the device.
In U.S. Patent Application Publication No. 20060102929 (Japanese Patent Application Publication No. 2004-200248), Okamoto et al. describe an HEMT in which the SiN passivation is a thin film that does not cover the gate electrode. A thicker silicon dioxide (SiO2) passivation film is formed on the SiN passivation film, also leaving the gate electrode uncovered. The top of the gate electrode is extended like a canopy over the passivation films, toward the drain, to reduce the concentration of the electric field at the edge of the gate electrode, thereby improving the source-drain breakdown voltage.
The devices described above are metal-semiconductor (MES) HEMTs, in which the gate electrode is in contact with the surface of the substrate, forming a Schottky junction that must be reverse-biased to prevent unwanted current flow from the gate electrode into the substrate. Even in the reverse-biased state, however, some gate current leaks through. An alternative to the MES-HEMT structure is the metal-insulator-semiconductor (MIS) HEMT structure, in which the gate electrode is separated from the substrate by a thin gate insulation film that greatly reduces the gate leakage current and enables the gate and substrate to be forward-biased.
The gate insulation film in a MIS-HEMT has also been reported to mitigate current collapse, as discussed by Ochiai et al. in ‘AlGaN/GaN Heterostructure Metal-Insulator-Semiconductor High-Electron-Mobility Transistors with Si3N4 Gate Insulator’, Japanese Journal of Applied Physics, Vol. 42, p. 2278 (2003). In a MIS-HEMT, however, it is difficult to use the field plate disclosed by Okamoto et al. to further mitigate current collapse, for the following reason.
In the fabrication of a MES-HEMT with a field plate, a passivation film is first formed on the entire surface of the substrate, then selectively removed to expose substrate areas on which the source, gate, and drain electrodes are formed. The field plate is then formed, covering the upper surface of the gate electrode, the side surface of the gate electrode facing the drain electrode, and the upper surface of the surface passivation film extending from the gate electrode toward the drain.
The effect of the field plate in reducing field concentration depends on the distance from the field plate to the substrate. The distance is equal to the thickness of the passivation film, or the combined thickness of the passivation films if there are multiple passivation films. The distance from the field plate to the substrate can be optimized by adjusting this thickness.
In the fabrication of a MIS-HEMT, the substrate is covered by a gate insulation film instead of a surface passivation film. The gate insulation film must be thin to provide an adequate field effect from the gate electrode; it cannot be thickened to adjust the distance from a field plate to the substrate. If the field plate is formed as an extension of the top of the gate electrode over the gate insulation film, then the distance from the field plate to the substrate cannot be adjusted to obtain the optimum reduction in field concentration. In general the field plate will be too close to the substrate.